A Study of the Dynamics of the Interaction of CH₄ with the (111) Surface of Rh and the (110) Surface of Ir

A study of the dynamics associated with the activated dissociative adsorption of CH4 on the (111) surface of Rh and the (110) surface of Ir has been performed in ultrahigh vacuum (UHV), using laser excitation and molecular beam techniques. Chapter 2 describes an experiment in which gas phase CH4 is laser excited to the ν3 excited vibrational state (8.63 kcal/mole) in a collisionless environment, and also, collisionally deactivated to the 2ν4 state (7.46 kcal/mole), in a He atmosphere. Hydrogen thermal desorption spectra from a Rh(111) surface exposed to the excited gas indicate that. within the experimental limits of detection, no enhancement of dissociative adsorption of CH4 is obtained by excitation to the ν3, 2ν4 (or ν4) vibrational states. Chapter 3 consists of the description of an ultrahigh vacuum-molecular beam apparatus constructed for the study of gas-surface dynamics. The ultrahigh vacuum chamber is a three level system of custom design. Level I is a load lock for introducing and retrieving the sample, to and from the UHV chamber. The molecular beam axis crosses the UHV chamber at Level II, and also crosses the axis of the ionizer of a quadrupole mass spectrometer detector. Low-energy electron diffraction optics are also mounted at Level II. Auger electron spectroscopy may be performed at Level III. An extended travel UHV sample manipulator has been designed and constructed, which allows translation of the sample to all three levels of the UHV system. The molecular beam line is of a nozzle source design, with three separate chambers which are pumped differentially. A chopper motor, for incident beam modulation, is mounted in the third beam chamber directly adjacent to the UHV chamber, and allows time-of-flight measurements. Chapter 4 describes an experiment in which the role of translational energy in the chemisorption of CH4 on Ir(110) is studied via the use of molecu1ar beams of CH4 seeded in H2 and He. A probability of dissociative adsorption below 10-4 is associated with a CH4 beam which has an average translational energy of approximately 10 kcal/mole. The results of Chapters 2 and 4 point to the existence of a barrier to dissociative adsorption of CH4 that is greater than 10 kcal/mole of total energy, on the surfaces studied. A combination of vibrational and translational activation of CH4 may be required for dissociative adsorption on Rh(111) and Ir (110).